Revert "Drop no-longer-necessary guard to javaSig"

This reverts commit 5cdb001.

Breaks in CI, here's a snippet:

    [info] Test dotty.tools.dotc.BootstrappedOnlyCompilationTests.posWithCompiler started
    -- Error: compiler/src/dotty/tools/dotc/transform/CapturedVars.scala:31:20 ---------------------------------------------
    31 |  private[this] var Captured: Store.Location[util.ReadOnlySet[Symbol]] = _
       |                    ^
       |compiler bug: created invalid generic signature for variable Captured in dotty.tools.dotc.transform.CapturedVars
       |signature: I
       |if this is reproducible, please report bug at https://github.com/lampepfl/dotty/issues
       |

compiler/src/dotty/tools/backend/jvm/BCodeHelpers.scala

@@ -847,12 +847,19 @@ trait BCodeHelpers extends BCodeIdiomatic with BytecodeWriters {
 
   private def getGenericSignatureHelper(sym: Symbol, owner: Symbol, memberTpe: Type)(using Context): Option[String] = {
     if (needsGenericSignature(sym)) {
-      val jsOpt = GenericSignatures.javaSig(sym, memberTpe)
-      if (ctx.settings.XverifySignatures.value) {
-        jsOpt.foreach(verifySignature(sym, _))
-      }
+      val erasedTypeSym = TypeErasure.fullErasure(sym.denot.info).typeSymbol
+      if (erasedTypeSym.isPrimitiveValueClass) {
+        // Suppress signatures for symbols whose types erase in the end to primitive
+        // value types. This is needed to fix #7416.
+        None
+      } else {
+        val jsOpt = GenericSignatures.javaSig(sym, memberTpe)
+        if (ctx.settings.XverifySignatures.value) {
+          jsOpt.foreach(verifySignature(sym, _))
+        }
 
-      jsOpt
+        jsOpt
+      }
     } else {
       None
     }